Method of separating toner particles from carrier particles in developer mixtures used in electrostatic printing

ABSTRACT

In an electrostatic-printing process employing a developer mixture comprised of electrically conductive carrier particles, and electrostatically attractable nonconductive toner particles, the carrier particles may be separated from the toner particles by immersing the developer mixture in a dilute electrolyte solution. The separation of the carrier particles from the toner particles permits the carrier particles to be reused with a variety of different colored toner particles.

United States Patent [111 3,572,501

[72] Inventor James Norton Brennan 2,638,416 5/1953 Wulkup 209/127UX Chicago, Ill. 2,772,991 12/1956 Insalaco l17/l7f5UX [21] Appl. No, 774,182 3,145,122 8/1964 Streich 209/129X 13g} f d 21 35 OTHER REFERENCES atente ar.

[ Assignee Continental can p y Inc. Optical Soc. of America, Vol. 38, No. 12, 993- 9 96, Dec.

New York, N.Y.

[54] METHOD OF SEPARATING TONER PARTICLES FROM CARRIER PARTICLES IN DEVELOPER MIXTURES USED IN ELECTROSTATIC PRINTING 4 Claims, No Drawings [52] U.S. Cl. 209/9, 117/ 17.5 [51] Int. Cl B03b l/04 [50] Field of Search 209/l--3.9, 127-130;117/l7.5;346/(lnquired);355/3,4, 17

[56] References Cited UNITED STATES PATENTS 2,290,6 86 7/1942 Morsfield 209/2 Primary Examiner-Frank W. Lutter Assistant Examiner-Robert Halper Anorneys- Paul Shapiro, Joseph E. Kerwin and William A.

Dittmann ABSTRACT: ln an electrostatic-printing process employing a developer mixture comprised of electrically conductive carrier particles, and electrostatically attractable nonconductive toner particles, the carrier particles may be separated from the toner particles by immersing the developer mixture in a dilute electrolyte solution. The separation of the carrier particles from the toner particles permits the carrier particles to be reused with a variety of difierent colored toner particles.

Patented March 30, 1971 I 3,572,501

4 Sheets-Sheet l INVENTOR. RICHARD M. QUINN BY DONNY W. CANDIOTO WWW/9; WW

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d e t n .w a P 4 Sheets-Sheet 4 an n o o N AT r s :p wk M mm: m9 wu UO Q QM ZOFOZDa EOE m INVENTOR. RICHARD M. QUINN BY DONNY W. CANDIOTO A T TOR/VEYS Mh'lll lfllll Oi SlEfAllA'lllNG TONER WtR'lllCLEb FROM @AlllillllER lPAll'llhClLlEfi KN DEVELOPER MTXTURES USED lll l ELEQTROSTATIIC lPlilllNG BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to electrostatic printing, and more particularly to an electrostatic printing process employing a developer mixture comprised of electrically conductive carrier particles and electrostatically attractable nonconductive toner particles.

2. The Prior Art Electrostatic-printing processes employing a developer mixture comprised of electrically conductive carrier particles and electrostatically attractable nonconductive toner particles for delivering an image to a substrate are well known to the art.

in an application for an electrostatic-printing system, Ser. No. 386,182, filed Jul. 30, 1964, now abandoned for John B. Kennedy, lr., which was assigned to a common assignee, there is described one method for electrostatic screen printing wherein a developer mixture of large conductive carrier particles having relatively small nonconductive toner particles adhered thereto by triboelectric forces is brought into contact with an electrically charged base electrode whereby the conductive carrier particles are charged therefrom so that a repulsion effect is established with sufiicient strength to cause the developer mixture to be repelled from the base electrode. A screen or stencil which has apertures arranged in a pattern is presented in the path of the developer mixture, the size of the apertures being selected so that the carrier particles are larger than the apertures while the toner is smaller than the apertures. The apertures in the screen mechanically stop the impelled carrier particles while the momentum of the toner causes these particles to detach from the carrier particles and pass through the screen and continue onward to impinge upon the surface of the article to be printed located above the screen and spaced therefrom, the areas of impingement of the toner particles on the substrate corresponding to the pattern on the screen. The image body thus formed is then fixed by the application of heat or vapor or the like fusion means which will cause the toner particles to be retained on the surface of the article. The carrier particles depleted of toner particles are removed from the printing area and are mixed with fresh toner to prepare the developer mixture which can be reemployed in the electrostatic-printing process.

When the article surface is of a conductive material, it is charged oppositely to the toner particles for controlling the paths thereof from the screen to the substrate. When the article surface is of a nonconductive nature, a backing or back electrode bearing a predetermined potential is placed at the rear of the article and on a side opposite to the side of the article facing the screen and base electrode.

Generally, the toner particles of the developer mixture contain black pigments and the image produced on a white substrate using these developer mixtures is black and white. lf multicolored prints are desired, developer mixtures having colored toner particles containing pigments of various colors are used. The carrier particles cannot be used interchangeably with different colored toner particles due to the formation of a thin surface film of the toner particles which remains attracted to the carrier particle surface even after the carrier particles are used in the electrostatic-printing process. When it is attempted to use carrier particles originally used with toner particles of one specific color with toner particles of a different color, the retention of a surface film of the original toner on the carrier particle causes a print of uncertain color to be delivered by the carrier particles to the surface of the article to be printed.

SUMMARY OF THE INVENTION in accordance with the present invention, carrier particles can be reused in developer mixture particles with toner particles of different colors to prepare multicolored prints by im mersing the carrier particles, from which the toner has been depleted in a printing operation in a dilute electrolyte to cause PREFERRED EMBODlMENTS Electrolyte solutions which may be employed in the process of the present invention to effect the separation of carrier and toner particles are dilute aqueous solutions, e.g., 0.10 percent to 10 percent of a strong electrolyte, i.e., strong acids, H 50 l-lCl, and bases NaOl-l, and KOl-l, and salts of these acids and bases, such as NaCl and KCll. Preferably, the electrolyte solution is one in which the electrolyte is a volatile material and substantially inert to the carrier particles, such as ammonium hydroxide.

To effect separation of the toner and carrier particles, the carrier particles are immersed in and washed with the electrolyte solution until the electrolyte wash solution appears clear and uncontaminated with toner material.

The words toner or toner particles" are employed herein to designate particles capable of adhesion to the carrier particles by triboelectric efiects, and capable of forming a pattern effect upon the substrate. The toner may be negatively or positively charged. Toner particles may be charged triboelectrically by mixing them with carrier particles. The polarity or charge the toner particles acquire is dependent upon the properties of both the toner and carrier material. Preferably, toners are used which, when mixed with iron or nickel carrier particles, exhibit a negative charge. Negatively charged toner particles are preferably of nonconductive material. Toner particles are generally comprised of pigmented thermoplastic resin particles having a particle size of from about 0.5 to about 30 microns and consist primarily of a finely divided uniform mixture of pigment in a nontacky styrene polymer resin, such as polystyrene, or a styrene-ethylacrylate copolymer. The styrene polymer resin is present in the toner composition in a predominating amount, i.e., at least about 50 percent of the entire composition. The pigment is present in the toner in a sufficient quantity to cause it to be highly colored whereby it will form a clearly visible image on the substrate on which it is electrostatically deposited. Thus, for example, in the case where lettering and the like is desired on corrugated paperboard, the pigment will be black pigment, such as carbon black or other minutely divided carbonaceous pigment. Pigments of different colors, such as red, blue and yellow may be used if a multicolored print is desired.

The carrier particles may be formed of any conductive material and include iron, nickel, aluminum, cobalt, copper, and their alloys. Carrier particle sizes of 2 to 250 microns are satisfactory for producing good, clear, dense prints.

The carrier particle should be present in the developer mixture in an amount sufficient to respond to the electric field and to carry sufficient amounts of toner through such field to the substrate. It has been found that the amount of toner mixed with the carrier particle can vary from about 0.5 to about 6 percent by weight of the carrier particles. Generally speaking, toner concentrations in the order of about 2 percent by weight prove satisfactory.

To illustrate the manner in which the invention may be carried out, the following example is given. it is to be understood, however, that the example is for the purpose of illustration, and the invention is not to be regarded as limited to any of the specific materials or conditions recited therein.

EXAMPLE 218 grams of nickel carrier particles having an average diameter of 3.8 microns were mixed with 4.37 grams of a carbon black pigmented styrene-ethyl acrylate toner particles having an average diameter of 0.6 microns to prepare a developer mixture which was used in printing a corrugated paper substrate in an electrostatic screen-printing process. After about 13,000 prints were made the developer mixture was washed with a 2 percent by weight aqueous ammonium hydroxide solution.

After this treatment, microscopic examination of the nickel carrier particles indicated that for all practical purposes, the toner had been completely removed from the carrier surface.

I claim:

1. In an electrostatic-printing process of the type wherein an image is produced on a substrate using a developer mixture composed of image-producing nonconductive toner particles adhered to the surface of larger, electrically conductive carrier particles by triboelectric forces, the toner particles being detached from the carrier particles at the time of printing and adhered to the substrate, the improvement which comprises removing the carrier particles depleted of toner particles from the printing area, and, before mixing the carrier particles with fresh toner particles to prepare the developer mixture, immersing the carrier particles depleted of toner particles, in a dilute electrolyte solution and washing with said solution to effect a discharge of the triboelectric forces between any remaining toner particles on the carrier particle surface so as to cause the release of the toner particles and obtain a carrier particle substantially free of toner particles said electrolyte being substantially inert to the carrier particles and being selected from the group consisting of inorganic acids, inorganic bases and the salts thereof.

2. The method of claim 1 wherein the toner is a mixture containing a styrene/ethyl acrylate copolymer resin, and carbon black.

3. The method of claim 1 wherein the carrier particles are composed of nickel.

4. The method of claim 1 wherein the electrolyte solution is a dilute solution of ammonium hydroxide. 

2. The method of claim 1 wherein the toner is a mixture containing a styrene/ethyl acrylate copolymer resin, and carbon black.
 3. The method of claim 1 wherein the carrier particles are composed of nickel.
 4. The method of claim 1 wherein the electrolyte solution is a dilute solution of ammonium hydroxide. 